Orthogonal Frequency Division Multiplexing, OFDM, is a modulation concept for use in wireless systems, e.g. 3GPP Long Term Evolution, LTE, IEEE WiMAX 802.16x, IEEE WiFi 802.11x, etc., which uses a large number of closely spaced orthogonal sub-carriers to carry data. OFDM allows flexible resource allocation over a wide bandwidth and more practical channel equalization algorithms as compared to more traditional single carrier systems. However, the OFDM signal is characterized by large fluctuations of its power envelope that result in occasional spikes in the power of the signal. Therefore, OFDM systems should be designed allowing large power margins, also known as power back-off, because the RF PA (Radio Frequency Power Amplifier), as well as other digital and analog components need to be dimensioned in order to handle the occasional power peaks of the OFDM signal.
Various metrics have been introduced for the quantification of the dispersion of the histogram of the power envelope of OFDM signals. The most common ones are the Peak to Average Power Ratio (PAPR) and the Cubic Metric (CM), as defined by Eq. 1 below. The Cubic Metric is currently employed as a reference measure in 3GPP. These metrics capture statistical properties of the signal and provide a preliminary indication of power derating, i.e., the power safety margin, i.e. back-off, that has to be kept in the power amplifier in order to reasonably limit the amount of distortion and out-of-band emission.
                    CM        =                  20          ⁢                                    log              10                        ⁡                          [                              rms                ⁡                                  [                                                            (                                                                                                                              y                            ⁡                                                          (                              t                              )                                                                                                                                                          rms                          ⁡                                                      (                                                          y                              ⁡                                                              (                                t                                )                                                                                      )                                                                                              )                                        3                                    ]                                            ]                                                          Eq        ⁢                                  ⁢        1            
For energy, cost, or space critical designs, e.g. in mobile devices, the power back-off margins required by OFDM may lead to an inefficient solution. Therefore, a modified OFDM modulation scheme, namely DFTS-OFDM (Discrete Fourier Transform Spread OFDM, also known as Single Carrier-OFDM or SC-OFDM), has been introduced and adopted by 3GPP LTE in order to improve the efficiency of uplink transmissions, i.e., reduce the PAPR and CM. In DFTS-OFDM a DFT (Discrete Fourier Transform) precoder is placed before the IDFT (Inverse Discrete Fourier Transform) modulator that is conventionally used for OFDM. Even though subcarrier mapping and equalization are still possible with roughly the same properties of conventional OFDM systems, DFTS-OFDM leverages a lower PAPR and CM than OFDM. This leads to lower power back-off margins in the PA and other components of the transmitter.
Another means to achieve better power efficiency in a mobile device is power control: the average power transmitted by a device is optionally reduced in order, e.g., to extend battery life or reduce interference. Power control also improves performance on a global network level, since a mobile device which does not use more power than necessary will cause less interference to the rest of the network.
The evolution of wireless communication systems envisages also the adoption of adaptive solutions for the multi-antenna transmission in the uplink. Multi-antenna transmission and reception is sometimes referred to as MIMO, or Multiple Input Multiple Output. This may be achieved by linearly combining on each antenna the signals generated by multiple independent DFTS-OFDM precoders. The simultaneous transmission of multiple independent data streams on the same bandwidth by taking advantage of multiple antennas is usually termed as Spatial Multiplexing (SM), while the technique of delivering each data stream on multiple transmit antennas by use of specific linear weighting factors is usually termed as Beam-Forming (BF). The combination of BF and SM potentially provides increased throughput for MIMO enabled devices and is foreseen as one of the major technical innovations for the uplink of forthcoming wireless telecommunication systems. In the following, a DFTS-OFDM system comprising of SM and BF will be shortly termed as MIMO DFTS-OFDM.
A convenient way to implement MIMO DFTS-OFDM is to insert a linear combiner, i.e. a precoding matrix W, before the DFT precoder. If the matrix W is correctly chosen, the incoming data streams are combined on the transmit antennas in order to optimally exploit the propagation properties of the wireless channel. The desired combiner W is usually selected by the receiver from a predefined set of combiners and it is signalled to the transmitter through a feedback channel. Such a predefined set of combiners, or precoding matrices, is referred to as a codebook. 3GPP has specified a standardized codebook for use in LTE systems. This standardized codebook will hereinafter be referred to as the 3GPP codebook. The transmitter updates the spatial combiner to the value that has been fed back at the latest possibility. The receiver then assumes that the signalled value of W is being employed at the transmitter and combines it with an estimation of the channel H to allow correct reception. The estimation of H at the receiver is made possible because the transmitter periodically transmits a predefined set of demodulation pilot tones.
FIG. 1 shows an exemplary MIMO DFTS-OFDM system, as described above. The system in FIG. 1 comprises two antennas, denoted “Antenna 1” and “Antenna 2”. Two data streams, denoted “data” are simultaneously transmitted on the same bandwidth. It should be noted that the use of two antennas and data streams is only exemplary. In general, the system may comprise any number Ntx of antennas, and any number Ns of data streams.
The system also includes a precoding unit 110, comprising coding and modulation units, a codeword to layer mapping unit, a spatial combiner W, and DFT precoders.
The two data streams are coded and modulated and fed to a spatial combining matrix W with dimensions 2×2 (spatial processing block). In the general case, the matrix W will have the dimensions Ntx×Ns, depending on the number of antennas and data streams, respectively. W distributes and weights the Ns incoming signals on the Ntx antennas. As mentioned above, the instantaneous value of W may be taken from a predefined codebook, and the specific codeword W to be used in the current transmission is based on a feedback previously received from the receiver.
The two streams that are produced by the spatial processing block are applied to corresponding two DFT precoders. Again, in the general case Ntx streams will be applied to Ntx DFT precoders. Each DFT precoder has size K and provides input to the corresponding IDFT modulator. Since the spatial processing and OFT are linear blocks, they might be equivalently swapped.
Each IDFT is followed by a power control block and a dedicated RF section, comprising also of a PA. The IDFT operates in a parallel way, i.e. on vectors. The P/S (parallel/serial) block serializes the output from the IDFT into a sequence for transmission. The CP (Cyclic Prefix) box is the cyclic prefix usually employed in OFDM systems. The cyclic prefix is a replica of part of the transmitted OFDM symbol. The CP box is useful for equalization at the receiver.
In an OFDM system using MIMO, the CM/PAPR may be too high even if DFTS-OFDM is used. This is particularly the case when the transmit antennas are strongly correlated, as will be further explained below. Transmit antennas are usually correlated when they are closely spaced compared to the wavelength of the transmitted signal, and when there are limited reflections in the environment surrounding the antennas. Transmit correlation is a common scenario in practice. Precoders that are suitable for correlated channels with strong correlation at the transmitter side often imply heavy mixing of the data streams on each antenna. Given a certain time and frequency resource, transmit correlation implies that the channels from different transmitting antennas to the same receiving antenna do not assume statistically independent values. The consequence is that the optimal spatial combiner W is usually a dense matrix, i.e. a matrix with no zero elements. This is reflected, e.g., in the combining matrices that have been standardized for the downlink of LTE, i.e. in the standardized 3GPP codebook.
However, the mixing of data streams implies a combination of independent signals. This has a negative effect on signal statistics, because according to the central limit theorem, the signal power of a large number of independent signals will approach a normal distribution. This is reflected by larger CM and PAPR. As has already been discussed, the consequence of high CM is a larger back-off in the Power Amplifier and consequently a less efficient design of the RF part.
A possible solution to reduce CM is to design a codebook of spatial combiners W such that the resulting CM is sufficiently moderate. This can be achieved, e.g., by nulling some elements in the combining matrix and avoid mixing of streams on the corresponding antennas. However, this usually comes at the price of reduced beamforming gain, especially for correlated channels, because the modified combining matrix diverges from the optimal dense matrix.
Another class of solutions that have been proposed in the literature is based on the processing of the signal to be modulated by, e.g., inserting suitable PAPR compensation tones or scrambling pattern. However, these techniques present several drawbacks: they are relatively complicated, they reduce spectral efficiency and they require dedicated signalling.
There is thus a need for a mechanism for improving power efficiency for precoded OFDM transmissions, which mitigates the drawbacks associated with the prior art.